Indexing in 3D printing

ABSTRACT

In an example implementation, a method of printing a three-dimensional (3D) object includes scanning a print bar in a first direction over a build platform of a 3D printer to deposit a liquid agent onto a layer of build powder. The print bar is then indexed in a second direction substantially orthogonal to the first direction before scanning the print bar back over the build platform in a third direction opposite the first direction to deposit additional liquid agent onto the layer of build powder.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/097,226, filed Oct. 26, 2018, which is a 371 application of PCTApplication No. PCT/US2016/043718, filed Jul. 22, 2016. The contents ofboth U.S. application Ser. No. 16/097,226 and PCT Application No.PCT/US2016/043718 are incorporated herein by reference in theirentirety.

BACKGROUND

Additive manufacturing processes can produce three-dimensional (3D)objects by providing a layer-by-layer accumulation and unification ofmaterial patterned from a digital model. In 3D printing, for example,digitally patterned portions of successive material layers can be joinedtogether by fusing, binding, or solidification through processesincluding melting, sintering, extrusion, and irradiation. The quality,strength, and functionality of objects produced by such systems can varydepending on the type of additive manufacturing technology used.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples will now be described with reference to the accompanyingdrawings, in which:

FIG. 1 shows a perspective view of an example 3D printing systemsuitable for implementing a multiple pass indexing method that enablesmore than one nozzle to print a liquid agent over a region of a buildplatform;

FIG. 2 shows a bottom view of an example of a print bar suitable toprovide platform-wide printing of a liquid agent onto a layer of powderon a build platform;

FIG. 3 shows an example printhead die that includes eight rows ofnozzles;

FIG. 4 a shows an example of multiple pass 3D printing with print barindexing;

FIG. 4 b shows an example of multiple pass 3D printing with buildplatform indexing;

FIG. 5 shows an example of several layers of a 3D object to demonstratethe effect of single pass indexing;

FIGS. 6 a and 6 b show examples of different multiple pass indexingschemes that may be suitable for multiple pass 3D printing and indexingin a 3D printing system;

FIGS. 7 and 8 are flow diagrams showing example methods of printing a 3Dobject.

Throughout the drawings, identical reference numbers designate similar,but not necessarily identical, elements.

DETAILED DESCRIPTION

In some examples of three-dimensional (3D) printing, 3D objects can beproduced in a 3D printing system (i.e., a 3D printer) by depositing andprocessing layers of build material, such as layers of powdered nylon,or polyamide. Each layer of build material (i.e., powder) can bedeposited and processed on a build platform within a system work space.The build platform, sometimes referred to as a powder bed, can be movedvertically downward to increase the height of the work space asadditional layers of powder are deposited and processed. Processing caninclude the selective application of a liquid agent onto layers of thepowder in areas where the powder is to be fused together. For example, aliquid fusing agent can be applied to cover a cross-sectional area of a3D object being printed, according to a 3D digital model. The fusingagent can coat the exterior surface of the powder and penetrate into alayer of powder. Processing can also include exposing the powder to afusing energy such as visible light radiation, infrared (IR) radiation,and ultraviolet radiation. The fusing agent deposited onto the buildpowder can absorb the radiation and convert it into thermal energy. Thethermal energy can fuse (i.e., melt and coalesce) those areas of thepowder to which the fusing agent has been applied. This process can berepeated with each layer of powder deposited into the work space untileach cross-sectional area is fused together to form a 3D object.

In some examples, 3D printing systems can implement inkjet technology todeposit the liquid fusing agent onto the layers of build powder. Forexample, a liquid agent dispenser can include a drop-on-demand printheadthat can be scanned over the build platform to selectively deliver afusing agent or other liquid onto a powder bed. Printheads can include,for example, thermal inkjet or piezoelectric inkjet printheads that havearrays of liquid ejection nozzles to jet the liquid agents onto thepowder. In some examples, multiple printheads can be aligned end-to-endalong the length of a print bar to enable a page-wide, or platform-wide,coverage of the powder bed through a single scan of the print bar overthe build platform.

3D printers that include powder beds and liquid jetting systems withscanning printheads are susceptible to various nozzle oriented defectsthat can result in reduced quality in printed objects, such as reducedsurface color quality and reduced part strength. For example, printheadnozzles can become blocked from airborne powder, other ambient debris,and/or dried agents. Other defects can include nozzles with dropejection directionality differences, nozzles with drop-weight anddrop-shape differences, and nozzles with differences in colorantconcentration. In some examples, color concentration differences betweennozzles can result from temperature variation across a single printheaddie during printing, and/or die-to-die variations when multiple die areprinting from a print bar, for example. In other examples, print barswith multiple aligned dies can exhibit die stitching defects that cancause irregular print patterns.

The use of redundant nozzles in a printhead can help to remedy somenozzle oriented defects. However, because printheads can have manythousands of nozzles, adding nozzle redundancy can increase the cost ofa printing system considerably. In addition, the use of redundantnozzles can include examining the performance of each nozzle on aprinthead to detect which nozzles are defective, and then employingredundant nozzles to remedy the defective nozzles. In addition to theadded costs associated with detecting defective nozzles, the additionaltime expended between printhead scanning cycles to examine thousands ofnozzles can have a noticeable adverse impact on printing speed.

Accordingly, in some examples described herein, a multiple pass indexing3D printer enables printing a 3D object by scanning a printhead, orprint bar, over a build platform multiple times and in different indexedpositions in order to deposit a liquid agent onto a layer of buildpowder. As used herein, ‘printhead’ can refer to an elongated print barhaving multiple printhead die aligned generally end-to-end to provide afixed array of printhead nozzles that can cover an entire width of aprint zone, such as the width of a powder bed. Indexing the printheadbetween multiple passes in an orthogonal direction relative to thescanning/printing direction of the printhead enables more than onenozzle to print over a region of the build platform. Regions that mayhave been missed or misprinted on a first pass by defective nozzles canbe covered by different nozzles on a subsequent pass. In some examples,single pass indexing can be implemented where the printhead is indexedbetween powder layers after a single pass per each layer. Single passindexing can decrease the time to print each layer as indexing theprinthead orthogonally and translating the printhead back to a startposition can both occur while a next layer of powder is being depositedonto the build platform.

Multiple printhead passes at different indexed positions can remedydefects such as missing nozzles, nozzles with drop ejectiondirectionality differences, nozzles with drop-weight and drop-shapedifferences, nozzles with color concentration differences, and diestitching defects. Each pass of the printhead can print a similar ordifferent loading of agents. The process can be repeated multiple timesper each layer of build powder while the printhead is in a differentorthogonal offset for each pass. In some examples, instead of indexingthe printhead, the build platform can be indexed. Indexing the printheadand/or build platform in this manner can reduce costs associated withusing physical redundant nozzles and improve build speed and throughputby reducing down time that may otherwise be expended for detectingnozzle defects and servicing the printhead. This solution mayadditionally reduce costs associated with color calibrations and scanneralignments.

In a particular example, a method of printing a 3D object includesscanning, or moving, a print bar in a first direction over a buildplatform of a 3D printer to deposit a liquid agent onto a layer of buildpowder. After scanning in the first direction, the print bar is indexedin a second direction that is substantially orthogonal to the firstdirection. The print bar is then scanned back over the build platform ina third direction that is opposite of the first direction to depositadditional liquid agent onto the layer of build powder.

In another example, a non-transitory machine-readable storage mediumstores instructions that when executed by a processor of athree-dimensional (3D) printer cause the 3D printer to apply a layer ofbuild powder onto a build platform of a 3D printer. The printer candeposit a liquid agent onto the powder with multiple passes of a printbar over the platform. During a first pass, the print bar can be passedover the platform with the print bar and platform in a first relativeposition to one another. After the first pass, the print bar andplatform can be indexed relative to one another to put the print bar andplatform into a second relative position to one another. During a secondpass, the print bar can pass over the platform with the print bar andplatform in the second relative position.

In another example, a device for printing 3D objects includes a buildplatform to receive build powder. The device also includes a print barto scan back and forth over the platform in multiple passes whileselectively depositing a liquid agent onto the build powder. A motorizedindexing arm is coupled to the print bar to index the print bar aftereach pass of the print bar over the platform. The indexing arm indexesthe print bar in a direction orthogonal to the scanning direction of theprint bar during each pass.

FIG. 1 shows a perspective view of an example three-dimensional (3D)printing system 100 suitable for implementing a multiple pass indexingmethod that enables more than one nozzle to print a liquid agent over aregion of a build platform. The example printing system 100 includes amoveable printing platform 102, or build platform 102 that can serve asa floor to a work space 104 in which a 3D object (not shown in FIG. 1 )can be printed. The work space 104 can include fixed walls 105(illustrated as front wall 105 a, side wall 105 b, back wall 105 c, sidewall 105 d) around the build platform 102. The fixed walls 105 andplatform 102 can contain a volume of powdered build material depositedlayer by layer into the work space 104 during printing of a 3D object.For purposes of this description and to help illustrate differentelements and functions of the 3D printing system 100, the front wall 105a of the work space 104 is shown as being transparent. During printing,a build volume within the work space 104 can include all or part of a 3Dobject formed by layers of powder that are processed with theapplication of liquid fusing agent and fusing energy (e.g., radiation).The build volume can also include non-processed powder that surroundsand supports the 3D object within the work space 104.

The build platform 102 is moveable within the work space 104 in anupward and downward direction as indicated by up arrow 106 and downarrow 108, respectively. When printing of a 3D object begins, the buildplatform 102 can be located in an upward position toward the top of thework space 104 as a first layer of powdered build material is depositedonto the platform 102 and processed. After a first layer of powder hasbeen processed, the platform 102 can move in a downward direction 108 asadditional layers of powdered build material are deposited onto theplatform 102 and processed.

The example 3D printing system 100 includes a supply of powdered buildmaterial 110, or powder. The build material, alternately referred toherein as “powder”, can comprise powdered material made from variousmaterials that are suitable for producing 3D objects. Such powderedmaterials can include, for example, polymers, glass, ceramics (e.g.,alumina, Al₂O₃), Hydroxyapatite, metals, and so on. The printing system100 can feed powder from the supply 110 into the work space 104 using aspreader 112 to controllably form the powder into layers over the buildplatform 102, and/or over other previously deposited layers of powder. Aspreader 112 can include, for example, a roller, a blade, or anothertype of material spreading device. Although not illustrated, in someexamples a carriage can be associated with the powder supply 110 and/orpowder spreader 112 to convey the supply and spreader over the buildplatform 102 during the forming of a layer of powder onto the platform.

The example 3D printing system 100 also includes a liquid agentdispenser 114. While other types of liquid dispensers are possible, theexample dispenser 114 shown and described herein comprises adrop-on-demand printhead 114 that can be scanned, or moved, over thebuild platform 102 to selectively deliver a fusing agent or other liquidonto a powder bed. Examples of drop-on-demand printheads include thermalinkjet and piezoelectric inkjet printheads that comprise an array ofliquid ejection nozzles. In some examples, the printhead 114 has alength dimension that enables it to span the full depth 116 of the buildplatform 102. Thus, a printhead 114, alternately referred to herein as aprint bar 114, can enable a page-wide or platform-wide coverage of thepowder bed through a single scan of the print bar over the buildplatform 102. In some examples, a 3D printing system 100 can includemore than one print bar 114.

FIG. 1 shows an example of the scanning motion (illustrated by directionarrow 120) of the print bar 114. In some examples a carriage (not shown)can be associated with the print bar 114 to convey the print bar 114over the build platform 102 during the application of a liquid agentonto a layer of powder on the platform 102. In some examples the printbar 114 can be coupled to a conveyor 140 that can be controlled to scanthe print bar 114 over the platform 102, as illustrated by thedashed-line print bar representation 122. As discussed in more detailbelow, the print bar 114 can be scanned back and forth over the platform102 in different indexed positions. Although not shown in the example ofFIG. 1 , during printing a portion of a 3D object would be presentwithin the work space 104 as the print bar 114 scans over the work spaceand ejects droplets 124 of a fusing agent or other liquid.

FIG. 2 shows a bottom view of an example of a print bar 114 suitable toprovide platform-wide printing of a liquid agent onto a layer of powderon the build platform 102. Platform-wide printing is enabled in part, bythe print bar 114 having multiple printhead die 117 positioned inparallel along the length 121 of the print bar 114 in an end-to-endalignment 119. As shown in the blow up view of FIG. 2 , the ends of themultiple printhead die 117 can be arranged in an overlapping alignmentof nozzles to help provide a seamless printing transition between themultiple die. With a continuous array of nozzles 115 spanning its length121, the print bar 114 can scan over the full width 118 and depth 116 ofthe build platform 102 as the nozzles jet droplets 124 of a fusingagent, colorant, or other liquid onto layers of powder within the workspace 104. The bottom view of the print bar 114 shown in FIG. 2 isprovided for the purpose of illustrating an example arrangement ofprinthead die 117 and nozzles 115 on the bottom side of the print bar,while the print bar 114 (122) in FIG. 1 is shown from a top perspectiveview with the nozzles 115 facing downward to eject liquid agent droplets124 over the build platform 102.

While the example printhead die shown in FIG. 2 include two rows ofnozzles 115, other nozzle configurations on a printhead die are possibleand contemplated. FIG. 3 shows an example printhead die 117 thatincludes eight rows 123 of nozzles. Such an arrangement can enablemultiple liquid agents, such as different ink colors and/or differentfusing agents, to be applied to a powder layer in a single pass over thebuild platform 102. In some examples, each of the two pairs of adjacentrows of nozzles 115 can be associated with a different fluid slot (notshown) formed in the substrate of the die 117. Each fluid slot cansupply a different liquid agent to nozzles in an associated pair ofadjacent rows of nozzles.

Examples of liquid agents suitable for ejection from nozzles in a printbar 114 can include water-based dispersions comprising a radiationabsorbing agent. The radiation absorbing agent can comprise, forexample, an infrared (IR) radiation absorber, a near infrared radiationabsorber, an ultraviolet radiation absorber, or a visible lightabsorber. In some examples, a fusing agent can be an ink-typeformulation as the radiation absorbing agent. In some examples, a fusingagent can be an ink or other liquid that absorbs energy in the IRspectrum but reflects energy in the visible light spectrum. Dye basedand pigment based colored inks are examples of inks that include visiblelight absorbing agent.

As shown in FIG. 1 , the example 3D printing system 100 also includes afusing energy source such as radiation source 126. The radiation source126 can be implemented in a variety of ways including, for example, as acuring lamp or as light emitting diodes (LEDs) to emit IR, near-IR, UV,or visible light, or as lasers with specific wavelengths. The radiationsource 126 can depend in part on the type of fusing agent and/or powderbeing used in the printing process. In different examples, the radiationsource 126 can be attached to a carriage (not shown) to be scannedacross the work space 104. The radiation source 126 can apply radiationR to layers of powder in the work space 104 to facilitate the heatingand fusing of the powder. In some examples, a fusing agent 124 can beselectively applied by print bar 114 to a layer of powder to enhance theabsorption of the radiation R and help convert the absorbed radiationinto thermal energy. In areas where fusing agent has been applied to thepowder, the absorbed radiation can heat the powder sufficiently to causefusing of the powder.

Referring still to FIG. 1 , the example 3D printing system 100additionally includes an example controller 128. The controller 128 cancontrol various operations of the printing system 100 to facilitate theprinting of 3D objects as generally described above, such as spreadingpowder into the work space 104, selectively applying fusing agent 124 toportions of the powder, and exposing the powder to radiation R. Inaddition, as described in more detail below, the controller 128 cancontrol the 3D printing system 100 to perform multiple passes of theprint bar 114 over the build platform 102 in different indexed positionsto deposit a liquid agent onto the powder. Indexing the print bar 114and/or build platform 102 to different positions between each print barpass enables more than one nozzle to print over a region of the buildplatform 102 and helps to provide coverage of regions that may have beenmissed or misprinted by a defective nozzle.

As shown in FIG. 1 , an example controller 128 can include a processor(CPU) 130 and a memory 132. The controller 128 may additionally includeother electronics (not shown) for communicating with and controllingvarious components of the 3D printing system 100. Such other electronicscan include, for example, discrete electronic components and/or an ASIC(application specific integrated circuit). Memory 132 can include bothvolatile (i.e., RAM) and nonvolatile memory components (e.g., ROM, harddisk, optical disc, CD-ROM, magnetic tape, flash memory, etc.). Thecomponents of memory 132 comprise non-transitory, machine-readable(e.g., computer/processor-readable) media that can provide for thestorage of machine-readable coded program instructions, data structures,program instruction modules, JDF (job definition format), 3MF formatteddata, and other data and/or instructions executable by a processor 130of the 3D printing system 100.

An example of executable instructions to be stored in memory 132 includeinstructions associated with a build module 134 and an indexing module136, while examples of stored data can include object data 138. Ingeneral, modules 134 and 136 include programming instructions executableby processor 130 to cause the 3D printing system 100 to performoperations related to printing 3D objects within a work space 104,including performing indexing a print bar 114 and/or build platform 102between multiple print bar 114 passes over the platform 102. Suchoperations can include, for example, the operations of methods 700 and800, described below with respect to FIGS. 7 and 8 , respectively.

In some examples, controller 128 can receive object data 138 from a hostsystem such as a computer. Object data 138 can represent, for example,object files defining 3D object models to be produced on the 3D printingsystem 100. Executing instructions from the build module 134, theprocessor 130 can generate print data for each cross-sectional slice ofa 3D object model from the object data 138. The print data can define,for example, each cross-sectional slice of a 3D object model, the liquidagents to be used to cover the build powder within each cross-sectionalslice, and how fusing energy is to be applied to fuse each layer ofpowder. The processor 130 can use the print data to control componentsof the printing system 100 to process each layer of powder. Thus, theobject data can be used to generate commands and/or command parametersfor controlling the distribution of build powder from a supply 110 ontothe build platform 102 by a spreader 112, the application of fusingagents by a print bar 114 onto layers of the powder, the application ofradiation by a radiation source 126 to the layers of powder, and so on.

The indexing module 136 includes further executable instructions toenable a processor 130 to control the 3D printing system 100 to performmultiple passes of the print bar 114 over the build platform 102 indifferent indexed positions to deposit a liquid agent onto a single orsame layer of powder. More specifically, indexing module instructionscan execute to control a conveyor 140 to scan the print bar 114 back andforth across the platform 102 to apply a liquid agent onto a powderlayer in multiple passes. The instructions can further control amotorized print bar indexing arm 142 to index the print bar indirections that are substantially orthogonal to the scanning directionof the print bar 114. The instructions can also control print data toadjust the distribution of liquid agent among individual nozzles and/orrows of nozzles. For example, because indexing the print bar 114 and/orbuild platform 102 to different positions between print bar passesenables more than one nozzle to print over a region of the buildplatform 102, the print data that controls the color, type, and/orloading of liquid agent to be deposited over that region can be shiftedaccordingly to different nozzles and/or rows of nozzles to correspondwith the direction of indexing. For example, indexing the print bar 114in a +Y direction (e.g., see description of FIG. 4 a ) may result inprint data being shifted to nozzles or rows of nozzles that are in the−Y direction on the print bar 114. Indexing of the print bar 114 canoccur at different times, such as before or after a first pass, beforeor after a second pass, and so on. In some examples, instead of indexingthe print bar 114, the indexing module instructions can control indexingof the build platform 102. Thus, the indexing module instructions cancontrol a motorized platform indexing arm 144 to index the buildplatform 102 in directions that are substantially orthogonal to thescanning direction of the print bar 114.

FIG. 4 a shows an example of multiple pass 3D printing with print barindexing. FIG. 4 a shows several top down views of an example buildplatform 102 during multiple passes of a print bar 114 over the platform102 to deposit a liquid agent onto a powder layer of a 3D object 146.The print bar 114 in FIG. 4 a is shown as transparent in order toillustrate the arrangement of printhead die 117 aligned on the bottomside of the print bar 114. In view (a) of FIG. 4 a , the print bar 114begins on the right side of the platform 102 in a first Y-coordinateindexed position with respect to the illustrated XY coordinate plane148. In a first pass, the print bar 114 is scanned over the buildplatform 102 in a first direction 150 from the right side to the leftside of the platform 102 (i.e., in a −X direction) to print a liquidagent onto a layer of a 3D object 146. As shown in view (a) of FIG. 4 a, in some examples a print defect 152 can occur due to one or multipledefective nozzles on the print bar 114. The print defect 152 isillustrated as a white line to indicate a region of the build platform102 or powder layer that was not printed on with liquid agent.

As shown in view (b) of FIG. 4 a , when the print bar 114 completes itsfirst pass over the build platform 102, the printing system 100 indexesthe print bar 114 in the +Y direction in preparation for a second passover the platform 102. In some examples, the system can index the printbar 114 in −Y direction. In any case, the indexing direction 154 of theprint bar 114 comprises a second direction 154 of movement that issubstantially orthogonal to the scanning or printing direction of theprint bar 114. Indexing the print bar 114 in the second direction 154moves the print bar 114 to a second Y-coordinate indexed position (i.e.,in XY coordinate plane 148) in preparation for a second pass over theplatform 102.

In some examples, the indexing offset amount, or distance, of print barmovement in the Y direction can be on the order of one half the lengthof a printhead die 117, to one full length of a printhead die 117. Otherindexing distances are also possible. In general, the index offsetdistance can be a minimum distance that moves nozzles far enough on the3D object 146 that print defects from defective nozzles on a first passcan be remedied on a subsequent pass, and so defects from the subsequentpass will be less apparent. As described below with respect to FIG. 6 ,different indexing schemes that define varying indexing directions anddistances, as well as varying multiple print bar pass patterns, can beimplemented and controlled through executable instructions from indexingmodule 136.

As shown in view (c) of FIG. 4 a , after the print bar 114 is indexed inthe (+) Y direction to a second indexed position, the print bar 114 isthen scanned in a third direction 156 over the build platform 102 todeposit a liquid agent onto the same layer of the 3D object in a secondpass. The third direction 156 is opposite the first direction 150. Asshown by the object 146 in view (c), the print defect 152 from defectivenozzles on the first pass has been resolved by working nozzles that haveapplied liquid agent to the region of the build platform 102 that wasnot printed on in the first pass. In some examples, the print bar 114can then be indexed in the −Y direction back to the first Y-coordinateposition (i.e., the starting position) in preparation to print ontoanother layer of powder added to the 3D object 146. Alternatively, theprocess can be repeated multiple times on the same layer of the 3Dobject 146, with the print bar 114 being indexed to a differentorthogonal offset per pass.

In some examples, rather than indexing the print bar 114, the printingsystem 100 may instead index the build platform 102 in the positive (+)or negative (−) Y directions. FIG. 4 b shows an example of multiple pass3D printing with build platform indexing. FIG. 4 b shows several topdown views of an example build platform 102 during multiple passes of aprint bar 114 over the platform 102 to deposit a liquid agent onto apowder layer of a 3D object 146. The print bar 114 in FIG. 4 b is shownas transparent in order to illustrate the arrangement of printhead die117 aligned on the bottom side of the print bar 114. In view (a) of FIG.4 b , the print bar 114 begins on the right side of the platform 102with the build platform 102 in a first Y-coordinate indexed positionwith respect to the illustrated XY coordinate plane 148.

In a first pass, the print bar 114 is scanned over the build platform102 in a first direction 158 from the right side to the left side of theplatform 102 (i.e., in a −X direction) to print a liquid agent onto alayer of a 3D object 146. As shown in view (a) of FIG. 4 b , in someexamples a print defect 152 can occur due to one or multiple defectivenozzles on the print bar 114. The print defect 152 is illustrated as awhite line to indicate a region of the build platform 102 or powderlayer that was not printed on with liquid agent.

As shown in view (b) of FIG. 4 b , when the print bar 114 completes itsfirst pass over the build platform 102, the printing system 100 indexesthe build platform 102 in the negative −Y direction in preparation for asecond pass of the print bar 114 over the platform 102. In someexamples, the system can index the platform 102 in a positive +Ydirection. The platform indexing direction 160 is substantiallyorthogonal to the scanning or printing direction of the print bar 114.Indexing the build platform 102 moves the platform 102 to a secondY-coordinate indexed position in preparation for a second pass of theprint bar 114 over the platform 102.

The index offset used when indexing the build platform 102 can besimilar to the index offset used when indexing the print bar 114. Asnoted above, in some examples the index offset can be on the order ofone half the length of a printhead die 117, to one full length of aprinthead die 117, with other offset values being possible. Indexing thebuild platform 102 shifts the 3D object 146 with respect to nozzles onthe print bar 114 so that different nozzles will be positioned to coverregions where there may be print defects caused by defective nozzles onthe first pass.

As shown in view (c) of FIG. 4 b , after the build platform 102 isindexed in the −Y direction to a second indexed position, the print bar114 is then scanned back over the platform 102 in a direction 162opposite the first scan direction 158 to deposit a liquid agent onto thesame layer of the 3D object in a second pass. As shown by the object 146in view (c), the print defect 152 from defective nozzles on the firstpass has been resolved by working nozzles that have applied liquid agentto the region of the build platform 102 that was not printed on in thefirst pass. In some examples, the build platform 102 can then be indexedin the +Y direction back to the first Y-coordinate position (i.e., thestarting position) in preparation to print onto another layer of powderadded to the 3D object 146. Alternatively, the process can be repeatedmultiple times on the same layer of the 3D object 146, with the platform102 being indexed to a different orthogonal offset per pass.

As mentioned above, in some examples single pass indexing can beimplemented where the printhead is indexed between powder layers after asingle pass per each layer. FIG. 5 shows an example of several layers ofa 3D object to demonstrate the effect of single pass indexing. In part(a) of FIG. 5 , each layer n, n+1, and n+2, has been printed with asingle pass of the print bar 114, but without indexing in betweenlayers. Each of three layers n, n+1, and n+2, in FIG. 5 , shows amissing region 170 (illustrated as regions 170 a, 170 b, 170 c) where adefective nozzle or nozzles missed printing onto the layer. It isapparent in part (a) of FIG. 5 , how the missing regions 170 are linedup due to no indexing. In part (b) of FIG. 5 , the print bar 114 hasbeen indexed by a distance 172 in between each layer. The missingregions 170 in part (b) of FIG. 5 are separated horizontally by theindexing distance 172. While the missing region 170 c in layer n+2appears to be empty, the missing region 170 b is darkened, whichindicates it has been partially or fully fused by the fusing of abovelayer n+2 and by heat conducting from layer n. Furthermore, the missingregion 170 a in layer n has been partially or fully fused by the fusingof layer n+1 directly above. Objects printed with single pass printingwill have greater strength if indexing between the layers isimplemented. This is because regions that do not get printed on due to anozzle defect can be partially or fully fused due to energy applied tothe next layer or heat conducting from the previous layer.

As mentioned above, executable instructions from an indexing module 136can control the implementation by the printing device 100 of a varietyof different indexing schemes. Different indexing schemes can definevarying indexing directions and indexing offsets of the print bar 114and/or the build platform 102, as well as the number and direction ofmultiple passes to be made over the platform 102 by the print bar 114.FIGS. 6 a and 6 b show some examples of different multiple pass indexingschemes that may be suitable for multiple pass 3D printing and indexingin a 3D printing system 100. The schemes will be described in terms ofindexing the print bar 114 rather than indexing the build platform 102.However, the described schemes may apply in the same or similar mannerwhen indexing the platform 102 as they do when indexing the print bar114. While several schemes are illustrated and described, it should beapparent that many other indexing schemes are possible and contemplatedherein.

Referring to FIGS. 6 a and 6 b , several examples of multiple passindexing schemes (a)-(g) are illustrated using direction arrows toindicate the XY movements within an XY coordinate plane 148 of a printbar 114 over a build platform 102 in a 3D printing system 100. DifferentY-coordinate index positions of the print bar 114, such as a first indexposition and a second index position, are illustrated using circlednumbers. For example, a circle with a number one indicates a first indexposition, and so on. Each scheme indicates multiple print bar passes andindexes that are to be made while printing or depositing liquid agentonto a single layer of powder of a 3D object. The print bar 114, buildplatform 102, and printing system 100 are not shown in FIGS. 6 a and 6b.

Referring to FIG. 6 a , in an example multiple pass indexing scheme (a),a first powder layer of a 3D object can be printed by starting the printbar in a first indexed position (circle number 1), and scanning in a −Xdirection over the build platform in a first pass. A first index in the+Y direction can then move the print bar into a second indexed position(circle number 2), followed by scanning the print bar in a +X directionback over the build platform. A second index in the −Y direction canthen return the print bar back to the first indexed position. In analternate scheme (b), the first index direction and the second indexdirection can be reversed. These schemes can be repeated for subsequentpowder layers of the 3D object.

In an example multiple pass indexing scheme (c), a first powder layer ofa 3D object can be printed by starting the print bar in a first indexedposition (circle number 1), and scanning in a −X direction over thebuild platform in a first pass. A first index in the +Y direction canthen move the print bar into a second indexed position (circle number2), followed by scanning the print bar in a +X direction back over thebuild platform. A next powder layer can then be applied to the 3Dobject, and the multiple pass indexing scheme (c) can continue inalternate ways. For example, for the next powder layer the print bar canbegin in the second indexed position where it left off from the firstpowder layer. The first pass over the next powder layer would then be inthe −X direction, followed by indexing in the −Y direction back to thefirst indexed position and a second pass in the +X direction.Alternatively, for the next powder layer, a return index in the −Ydirection can return the print bar back to the first indexed positionprior to the first pass in the −X direction.

In an example multiple pass indexing scheme (d), a first powder layer ofa 3D object can be printed by starting the print bar in a first indexedposition (circle number 1), and scanning in a −X direction over thebuild platform in a first pass. A first index in the −Y direction canthen move the print bar into a second indexed position (circle number2), followed by a second pass in the +X direction. A second index againin the −Y direction can then move the print bar into a third indexedposition (circle number 3), followed by a third pass in the −Xdirection. A return pass in the +X direction can then be made to returnthe print bar to the starting side of the build platform. A next powderlayer can then be applied to the 3D object, and the multiple passindexing scheme (d) can continue in alternate ways. For example, for thenext powder layer the print bar can begin in the third indexed positionwhere it left off from the first powder layer. The first pass over thenext powder layer would then be in the −X direction, followed byindexing in the +Y direction back to the second indexed position and asecond pass in the +X direction, and so on. Alternatively, for the nextpowder layer a return index in the +Y direction can return the print barback to the first indexed position prior to the first scan in the −Xdirection. While one example scheme is described for the next powderlayer, other schemes for the next powder layer and subsequent powderlayers are possible and are contemplated herein.

Referring now to FIG. 6 b , in an example multiple pass indexing scheme(e), four passes are made over a first powder layer (n) on a buildplatform while indexing the print bar in a −Y direction following eachpass. After the four passes are complete, a next powder layer (n+1) isapplied to the platform and 4 passes are again made over the platform.The first pass over the n+1 powder layer begins in position 4, where thelast pass over the first powder layer n ended. The four passes and theindexing over the n+1 layer proceed in just the opposite directions asthe four passes and indexing over the first layer n. As noted above,other example schemes are possible and contemplated. For example, priorto beginning the first pass over the n+1 powder layer, the print barmight be indexed in the +Y direction back to position 1. Schemes (f) and(g) of FIG. 6 b , illustrate examples of single pass indexing asdescribed above with regard to FIG. 5 .

FIGS. 7 and 8 are flow diagrams showing example methods 700 and 800, ofprinting a three-dimensional (3D) object. Methods 700 and 800 areassociated with examples discussed above with regard to FIGS. 1-6 , anddetails of the operations shown in methods 700 and 800 can be found inthe related discussion of such examples. The operations of methods 700and 800 may be embodied as programming instructions stored on anon-transitory, machine-readable (e.g., computer/processor-readable)medium, such as memory 132 shown in FIG. 1 . In some examples,implementing the operations of methods 700 and 800 can be achieved by aprocessor, such as a processor 130 of FIG. 1 , reading and executing theprogramming instructions stored in a memory 132. In some examples,implementing the operations of methods 700 and 800 can be achieved usingan ASIC and/or other hardware components alone or in combination withprogramming instructions executable by a processor 130.

The methods 700 and 800 may include more than one implementation, anddifferent implementations of methods 700 and 800 may not employ everyoperation presented in the respective flow diagrams of FIGS. 7 and 8 .Therefore, while the operations of methods 700 and 800 are presented ina particular order within their respective flow diagrams, the order oftheir presentations is not intended to be a limitation as to the orderin which the operations may actually be implemented, or as to whetherall of the operations may be implemented. For example, oneimplementation of method 800 might be achieved through the performanceof a number of initial operations, without performing one or moresubsequent operations, while another implementation of method 800 mightbe achieved through the performance of all of the operations.

Referring now to the flow diagram of FIG. 7 , an example method 700 ofprinting a three-dimensional (3D) object begins at block 702 withscanning a print bar in a first direction over a build platform of a 3Dprinter to deposit a liquid agent onto a layer of build powder. In someexamples, the method can include first depositing the layer of buildpowder onto the build platform. As shown in block 704, the method caninclude indexing the print bar in a second direction substantiallyorthogonal to the first direction. In some examples, the indexing caninclude indexing the build platform (i.e., instead or, or in addition toindexing the print bar) in a direction substantially orthogonal to thefirst direction, as shown at block 706. In some examples, indexing theprint bar can include moving the print bar from a first indexed positionto a second indexed position, as shown at block 708. As shown at block710, in some examples indexing the print bar can include moving theprint bar a distance of a partial length of one printhead die on theprint bar, or a distance of a full length of one printhead die on theprint bar. Moving the print bar a partial length of one printhead diecan include moving the print bar a distance of half of a printhead die,as shown at block 712. Is some examples, the method can include applyingfusing energy to the build powder on the platform after liquid agent hasbeen applied to the powder. The method 700 can continue as shown atblock 714, with scanning the print bar back over the build platform in athird direction opposite the first direction to deposit additionalliquid agent onto the layer of build powder.

As shown at block 716, with the print bar still in the second indexedposition, the print bar can be scanned in the first direction over thebuild platform to deposit a liquid agent onto a next layer of buildpowder. In some examples, the method can include depositing the nextlayer of build powder onto the build platform. As shown at blocks 718and 720, respectively, the method can then include indexing the printbar in a fourth direction opposite the second direction to move theprint bar back to the first indexed position, and scanning the print barback over the build platform in the third direction to depositadditional liquid agent onto the next layer of build powder.

Referring now to the flow diagram of FIG. 8 , an example method 800 ofprinting a three-dimensional (3D) object begins at block 802 withapplying a layer of build powder onto a build platform of a 3D printer.The method 800 continues at block 804 with depositing liquid agent ontothe build powder with multiple passes of the print bar over theplatform. During a first pass, the print bar can pass over the platformwith the print bar and platform in a first relative position to oneanother, as shown at block 806. After the first pass, the print bar andplatform can be indexed relative to one another to put the print bar andplatform into a second relative position to one another, as shown atblock 808. In some examples, indexing the print bar and platformrelative to one another can include indexing the print bar while leavingthe platform in a current position, as shown at block 810. In someexamples, indexing the print bar and platform relative to one anothercan include indexing the platform while leaving the print bar in acurrent position, as shown at block 812.

As shown at block 814, the method can continue with passing the printbar over the platform with the print bar and platform in the secondrelative position during a second pass. A next layer of build powder canthen be applied onto the build platform, as shown at block 816. With theprint bar and platform still in the second relative position, the printbar can be passed over the platform to deposit a liquid agent onto thenext layer of build powder, as shown at block 818. As shown at block 820and 822, respectively, the method 800 can also include indexing theprint bar and platform relative to one another to put the print bar andplatform back into the first relative position to one another, and withthe print bar and platform in the first relative position, passing theprint bar over the platform to deposit additional liquid agent onto thenext layer of build powder.

What is claimed is:
 1. A three-dimensional (3D) printing systemcomprising: a build platform to receive build powder; a platform-wideprint bar to scan back and forth over the platform; and, a processorprogrammed with instructions from an indexing module to scan the printbar in a single pass and a first direction over the platform to enableselective printing of a liquid agent onto an entire layer of buildpowder, to index the print bar in a second direction substantiallyorthogonal to the first direction, and to scan the print bar back overthe platform in a single pass and a third direction opposite the firstdirection to enable selective printing of additional liquid agent ontothe layer of build powder.
 2. The 3D printing system as in claim 1,further comprising: a print bar motorized indexing arm coupled to theprint bar and controllable by the processor to index the print bar aftereach pass in a direction orthogonal to a direction of the scan.
 3. The3D printing system as in claim 1, further comprising: a platformmotorized indexing arm coupled to the platform to index the platformafter each pass in a direction orthogonal to the direction of the scan.4. The 3D printing system as in claim 2, wherein the processor isprogrammed with further instructions to control an index offset and anindex direction of the print bar motorized indexing arm.
 5. The 3Dprinting system as in claim 3, wherein the processor is programmed withfurther instructions to control an index offset and an index directionof the platform motorized indexing arm.
 6. The 3D printing system as inclaim 1, wherein the processor is programmed with further instructionsto control the print bar to print a similar loading or a differentloading of the liquid agent with each pass of the print bar over theplatform.
 7. The 3D printing system as in claim 1, further comprising: aspreader to form build powder into layers over the platform; and, afusing energy source to apply fusing energy to the layers; wherein theprocessor is programmed with further instructions to control thespreader to form each layer of build powder onto the platform prior toscanning the print bar in the first direction, and to control the fusingenergy source to apply fusing energy to the build powder of each layerafter scanning the print bar in the third direction.